Well, in a quick explanation with out actually trying to do it. I would try the following. First-you need your background that you want to reveal. They use a white background with a light red gradient. Second- You need some growing flower/vines video with a alpha channel. The best way to do this, so your vines/flowers will have a alpha channel background, would be to do it yourself.
If you don't know how too do this, here is a great tutorial. http://www.creativecow.net/articles/hansen_jaysen/growing/growing_things.ht... Third-Once your vine video is created you need to turn your vines pure white. Try using the effect>color correction>colorama or effect>color correction>exposer and then place the growing video on top of the background, (make sure it is directly above the background). Forth- On the background change the track matte (trkmat) to "Luma Matte" - If you don't see it on the layer, try hitting the F4 button. Fifth- I would Make a duplicate of your back ground (make sure it does not have a track matte on) and place it at the bottom of the layers. The double click on the Rectangle mask, Feather about 50px, bring the expansion down to about -180 (or more, untill the background it gone) and Keyframe the expansion up, untill it reveals the background.
Now if you are doing this all from scratch, don't expect this to be a quick project.
Hope it all works well.